It would cost $9 million — six times more than advocates estimate — to equip all 600 elementary schools with alarms, a top schools official said Thursday in advocating a more piecemeal approach.

“It’s a large system, a complicated system. Principals have their own views, and we need to take their views into account,” Deputy Schools Chancellor Kathleen Grimm testified at a City Council hearing.

Councilman Robert Cornegy ­(D-Brooklyn), a father of six children who is sponsoring a bill to mandate the alarms, rejected Grimm’s alternative as inadequate. “The first priority for us cannot be on principals, it has to be on our children,” he declared.

The alarms became a priority for ­advocates after autistic 14-year-old Avonte Oquendo slipped out of his Queens school last year and was later found dead. Eight other students between the ages of 4 and 8 have gone missing from school buildings this school year but were found safe.